By Rachel Feltman
Fear is one of our most basic evolutionary instincts, a sudden physical jolt to help us react to danger more quickly. In the modern world, fear often seems excessive -- in the absence of wild animals to flee, we're left screaming over roller coasters and scary movies. But for at least one woman, fear is unobtainable. And while she lives a normal life, her fearlessness is actually a handicap.
The researchers who study her keep her closely guarded, using the code-name "SM" when publishing papers about her brave brainpower. And until this year, she'd never been interviewed.
"Tell me what fear is," Tranel began.
"Well, that's what I'm trying to -- to be honest, I truly have no clue," SM said, her voice raspy. That's actually a symptom of the condition that stole fear from her. Urbach-Wieth disease, which is characterized by a hoarse voice, small bumps around the eyes, and calcium deposits in the brain is rare in its own right -- only 400 people on the planet are known to have it -- but in SM's case, some of those brain-deposits happened to take over her amygdalae.
These almond-shaped structures deep inside the brain are crucial to human fear response. And in SM's case, they've been totally calcified since she was a young woman. Now in her 40s, her fear-center is as good as gone.
"It's a little bit as if you would go to this region and literally scoop it out," Antonio Damasio, another neuroscientist who studies SM, told "Invisibilia" hosts Lulu Miller and Alix Spiegel.

By Tia Ghose
Being around strangers can cause people stress and, in turn, make them less able to feel others' pain, new research suggests.
But giving people a drug that blocks the body's stress response can restore that sense of empathy, scientists said. What's more, the same effect shows up in both humans and mice.
"In some sense, we've figured out what to do about increasing empathy as a practical matter," said Jeffrey Mogil, a neuroscientist at McGill University in Montreal. "We've figured out what stops it from happening and, therefore, the solution to make it happen more between strangers."
Decreasing stress by doing a shared activity could be a simple way to increase empathy between people who don't know each other, the findings suggest.
Past studies had found that mice seemed to feel the pain of familiar mice but were less responsive to foreign mice. Other studies found that, in both humans and mice, stress levels tended to rise around strangers.
To see how stress and empathy are connected, Mogil and his colleagues placed two mice together in a cage, then inflicted a painful stimulus on one of them. When the mice were cage mates, the unaffected mouse showed more signs of pain than when they were strangers. But when the team gave the mice a drug called metyrapone, which blocks the formation of the stress hormone cortisol, the mice responded equally to the strangers' pain.